Optical system having a holographic optical element

ABSTRACT

An optical laser system wherein a holographic optical element (HOE) replaces a bulky feedback system comprising a large number of optical element. The feedback system is adjusted so that the laser device and the feedback system cooperate to select a state having a high temporal and/or spatial coherence, and the optical properties of the optical elements are recorded into the HOE. When the feedback system is removed the HOE reproduces the properties of the optical elements of the feedback system. The laser system is compact in size, cheap to manufacture, has high mechanical stability, and is less fragile than ordinary feedback systems. The laser system may be used in environments, such as the printing industry, which normally do not permit an ordinary feedback system, e.g. due to mechanical vibrations or misalignment due to temperature variations. A number of centre frequencies may be multiplexed into the HOE. May be mass produced. Furthermore, a method of producing such an optical laser system.

FIELD OF THE INVENTION

The present invention relates to an optical system, in particular acompact optical system comprising a holographic optical element and tothe use of a compact optical system in a laser system so as to provide acompact laser system with good spatial and temporal coherence.

BACKGROUND OF THE INVENTION

From WO 98/56087, it is known to use phase conjugate feedback in a lasersystem in order to obtain a highly coherent, possibly single mode,output light beam. However, the laser system disclosed in WO 98/56087requires a number of optical elements. Such optical elements areexpensive, especially if one of the optical elements comprises ananisotropic ferroelectric crystal, such as a BaTiO₃ crystal.Furthermore, BaTiO₃ crystals have a phase transition near roomtemperature and consequently they are very fragile and therefore need tobe handled with much care. In addition to this, using a large number ofoptical elements requires a precise alignment of the elements, and italso results in a bulky laser system which is sensitive to mechanicalvibrations. The alignment may be difficult to obtain and especially topreserve outside of a laboratory, and, furthermore, the bulky lasersystem is not very convenient for the user.

‘In Holographic optical head for compact disk applications’, OpticalEngineering, Vol. 28(6), pp 650-653, June 1989, is disclosed an opticalhead for a CD player based on the holographic optical element andlaser/detector hybrid technology. The holographic optical elementsdisclosed in this document are adapted to replace a number of refractiveelements, such as lenses, beamsplitters, and diffraction gratings, oroptionally a simple mirror. The document further discloses a method offabrication of computer-generated holographic optical elements.

A number of references describe the use of a holographic optical element(HOE) for replacing one or more optical elements. However, none of thesereferences disclose using a HOE for injecting the beam back into thelaser in order to improve the spatial and/or temporal properties of theoutput beam. The HOE is rather used for compensating for ‘beam defects’,such as astigmatism, after the beam has been output from the opticalsystem.

WO 99/57579 discloses a method for designing and constructing miniatureoptical systems and devices employing light diffractive optical elements(DOEs) for modifying the size and shape of laser beams produced fromcommercial-grade laser diodes. The DOEs may be implemented asholographic optical elements (HOEs). The DOE compensates for ‘beamdefects’, such as astigmatism, of a beam emitted from a laser system.The beam is not injected back into the laser.

U.S. Pat. No. 6,018,402 discloses the use of a holographic opticalelement (HOE) to reconstruct optical elements typically used tophaseencode an object beam emanating from a spatial light modulator(SLM). The HOE replaces the complicated phase mask and conventionalfour-F lens system arrangement typically used to phase-encode anamplitude-encoded object beam emanating from the SLM. Thus, the HOE isin this case used for converting and transforming laser light from onestate to another.

‘Recent studies of miniaturization of optical disk pickups in Japan’ byHiroshi Nishihara, Proceedings of the SPIE—The International Society forOptical Engineering, 1990, USA, vol. 1248, pages 88-95, XP001029989,describes methods for improving pickups for compact disk players. Aholographic optical element may be used for improving the output powerof the diode laser. Thus, the temporal and/or spatial properties of thebeam are not affected by the HOE.

It is, furthermore, known to produce holographic optical elements, e.g.for producing bright, sharp, three-dimensional images.

In contrast to the applications of HOE and DOE mentioned above thepresent invention deals with the improvement of the spatial and temporalcoherence of high power laser diodes when the HOE or DOE is used tofeedback light into the active region of the high power laser diode.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide a laser system whichis compact and cheap to manufacture, which is less fragile than knownlaser systems, and which is less sensitive to misalignment of theoptical elements which may be caused, e.g., by mechanical vibrations,temperature changes, etc.

It is a further object to provide methods of manufacturing a lasersystem having the properties described above.

It is an even further object to provide an optical system having asimple optical component which provides a passive feedback to the lasersystem.

It is an even further object to provide an optical system in which theoutput beam has been subject to losses which are substantially smallerthan losses caused by known feedback systems.

It is an even further object to provide a feedback system with a highmechanical stability.

It is an even further object to provide an optical system which iscapable of producing an output beam having better spatial and/ortemporal properties than the output beam from known systems.

It is a very important object of the invention to provide a diode lasersystem with high output power, in the range of 1 W to 1000 W, which canbe focused to a small diffraction limited spot.

According to a first aspect of the present invention there is providedan optical system for emission of an output light beam, wherein aholographic optical element reproduces the optical properties of aplurality of optical elements, the plurality of optical elements forminga feedback system being adapted to cooperate with a laser device toselect a high temporal and/or spatial coherent state of the laserdevice.

The output light beam is emitted from the optical system, i.e. it isavailable for other purposes. That is, the output light beam may be usedas a source of electromagnetic radiation. Thus, the output light beam isan electromagnetic output beam, such as a light beam, an ultravioletbeam, a microwave beam, an X-ray beam, or any other suitable kind ofelectromagnetic beam.

The optical properties of the optical elements may be any suitable kindof optical properties, such as refractive index, reflectivity, selectionof, e.g., frequencies or spatial modes, frequency doubling, etc.,depending on which kinds of optical elements are used.

The plurality of optical elements which may be reproduced by theholographic optical element form a feedback system being adapted tocooperate with a laser device to select a high temporal and/or spatialcoherent state of the laser device.

The laser device, thus, being adapted to supply a first light beam tothe optical system, and the laser device and the holographic opticalelement reproducing the optical properties of the optical system maythen cooperate to select a high temporal and/or spatial coherent stateof the laser device.

The feedback system used to improve the coherence properties of lasersystems may be an optical system reflecting at least a part of the firstlight beam emitted from the laser device back into the laser device. Incooperation, the feedback system and the laser device then force thelaser device to emit laser radiation with a high temporal and/or spatialcoherence. When a high temporal coherent state is selected, the outputlight beam of the system, thus, comprises radiation within a very narrowfrequency range, and, preferably, the output light beam substantiallycomprises only a single frequency. When a high spatial coherent state isselected the output light beam may be focused to a small spot size ofthe order of a wavelength. This is an important property for a largenumber of applications.

Preferably, at least one of the plurality of optical elements isselected from the group consisting of:

-   -   spatial filters,    -   gratings,    -   mirrors,    -   Fabry Perot etalons,    -   frequency filters.

It is a great advantage of the present invention that a HOE is used as afeedback system. Thus, a HOE is substantially less expensive than alarge number of optical elements.

Furthermore, the size of the entire optical system is substantiallyreduced, and the system is much more stable, e.g. with respect tovibrations, misalignments, etc. Thus, the HOE may be attached to thelaser facet itself, thereby substantially improving the mechanicalstability properties of the optical system. Also, the HOE provides apassive feedback system as opposed to the active feedback systemprovided by a feedback system comprising non-linear optical components.This is a great advantage because in comparison with the active feedbackthe passive feedback only uses low cost elements. Finally, a feedbacksystem being represented by a HOE may introduce fewer losses than afeedback system comprising the actual optical components which the HOErepresents. This is because it is possible to let the HOE reproduceother optical properties of the individual optical component withoutreproducing the loss characteristics of that component. In case the HOErepresent a large number of optical components, this is a very importantadvantage since the entire system may introduce very heavy losses.

A spatial filter may, e.g., be an aperture, a slit, a pinhole or anyother suitable kind of spatial filter. The optical properties of aspatial filter which may be reproduced by the holographic opticalelement in this case preferably comprise selection of specific modes orfrequencies.

In case one of the optical elements is a grating, the optical propertiesto be reproduced by the holographic optical element preferably comprisea frequency selectivity.

In case one of the optical elements is a mirror, the optical propertiesto be reproduced by the holographic optical element preferably comprisethe reflective properties of the mirror, such as a frequency dependentreflectivity, i.e. the reflectivity of the mirror at a certainfrequency, the tilt angle of the mirror, and/or any focusing propertiesof the mirror. The mirror may be a plane mirror, a parabolic mirror, aspherical mirror, a mirror with a spatial selectivity, or a mirrorhaving any other suitable shape.

A frequency filter may e.g. comprise a grating, such as a diffractiveoptical element, preferably in combination with a spatial filter, suchas an aperture or a slit, or it may be a filter which transmitselectromagnetic radiation within a specific frequency range and reflectsor absorbs any other frequencies. It may comprise an interferencefilter, an absorbance filter, such as a semiconductor doped glass, anetalon, such as a Fabry Perot element, a prism etc. The frequency filtermay be adapted to select a range of frequencies, preferably a narrowrange, most preferably a single frequency. In case one of the opticalelements is a frequency filter, the optical properties to be reproducedby the holographic optical element preferably comprise frequencyselection, transmission/reflection/absorption properties, selection ofmodes, reflective properties, refractive index, transmission properties,reflectivity, interference properties (destructive and/or constructive),or any other suitable properties of the frequency filter.

According to a second aspect the present invention further provides amethod of producing an optical system for emission of an output lightbeam, the method comprising the steps of:

-   -   inserting a holographic recording material into an external        cavity formed between a laser device and a feedback system, said        feedback system comprising a plurality of optical elements,    -   emitting, by means of the laser device, a first light beam, at        least part of said first light beam illuminating at least part        of the feedback system via said holographic recording material,    -   adjusting the feedback system so that the laser device and the        feedback system cooperate to select a state having a high        temporal and/or spatial coherence,    -   recording a holographic optical element in the holographic        recording material,    -   developing the holographic optical element so that the        holographic optical element is adapted to reproduce the optical        properties of the plurality of optical element when said        feedback system is removed, and    -   removing the feedback system.

The holographic recording material may be any material with aphotosensitive refractive index and/or absorption coefficient forexample a dichromatic gelatine, a Silver Bromide (AgBr) solution, photoresist, and/or a photorefractive medium.

The laser device may be a single laser, e.g. a gas laser, asemiconductor laser, a broad area laser, a superluminescent laser diode,a dye laser, a Nd-YAG laser, an argon ion laser, a titanium sapphirelaser, an F-center laser, or any other suitable kind of laser. It mayalso be an array of lasers, said lasers being of any of the typesmentioned above.

The feedback system is defined above.

Adjusting the feedback system may comprise adjusting one or more of theoptical elements forming the feedback system in such a way that a statehaving a high temporal and/or spatial coherency is selected. Theadjusting step may, furthermore, comprise alignment of the opticalelements. Additionally or alternatively, the adjusting step may compriseadjusting one or more optical element(s) in such a way that, e.g., acertain frequency, a certain spatial mode, etc. is selected. This may,for example, be done in the following way.

In a preferred embodiment of the invention, the feedback systemcomprises a reflector, the reflector being adapted to reflect at least apart of the first light beam emitted by the laser device back into thelaser device. The free running laser emits a large number of spatialmodes. By using spatial filtering for example in the Fourier plane, e.g.by means of one or more spatial filter(s), such as aperture(s), slit(s),pinhole(s), etc., the system is adjusted to emit laser light having ahigh temporal and/or spatial coherency. The feedback system may,furthermore, comprise a grating or an etalon so that the frequency ofthe first light beam may be tuned by tilting the grating or the etalon.It is, thus, possible to adjust the system to emit an output light beamhaving, e.g., a certain spatial mode, frequency, etc., depending on theoptical elements being provided in the feedback system.

When the feedback system has been adjusted so that the output light beamhas the desired properties, a holographic optical element having theseproperties is recorded in the holographic recording material positionedbetween the laser device and the feedback system. When the holographicoptical element is subsequently developed, it will thus be adapted toreproduce the optical properties of the elements in the feedback system.The feedback system may then be removed and the holographic opticalelement will act as the feedback system, i.e. the output beam will havethe same desired properties which the feedback system was adjusted toprovide. Of course, the recorded and developed holographic opticalelement itself may not afterwards be adjusted as the feedback system,thus, limiting the flexibility of the system. However, it is anadvantage of the system according to the present invention that thesystem provided is substantially non-sensitive to misalignments due tovibrations, temperature variations, etc. It is a further advantage thata rather bulky, expensive, and fragile feedback system may be replacedby the compact, cheap, and reliable holographic optical element. Theseadvantages makes the system very advantageous for commercial purposes.It is, thus, possible to use to system under conditions which arenormally not suited for a feedback system comprising a large number offragile optical components, e.g. in an environment introducingvibrations, temperature variations, a dirty environment, etc. It ispossible to manufacture HOEs under ideal conditions and subsequentlyposition the HOEs in optical systems under less ideal conditions. Thus,an ideal feedback system is provided even though the environment is notsuited for such a feedback system. Furthermore, the price of the entiresystem will be sufficiently low to attract potential customers. Thesystem may, thus, advantageously be used, e.g., in the printingindustry, medical applications, or telecommunication.

In a very preferred embodiment laser systems having a HOE replacing afeedback system may be mass produced by recording one HOE by the methoddescribed above, and subsequently reproduce this HOE. The reproducedHOEs may then be positioned in laser systems having similar properties.It should be noted that the HOE in each case should be positioned in thelaser system in a position corresponding to the position in which theoriginal HOE was recorded in order for the HOE to properly reproduce thefeedback system. Such mass produced laser systems are very advantageousfrom a commercial point of view since they are very cheap tomanufacture, and the price therefore will be acceptable for potentialcustomers.

The method may comprise the steps of, for each of the optical elements:

-   -   adjusting the feedback system so that the laser device and the        feedback system cooperate to select a state having a high        temporal and/or spatial coherency,    -   recording a holographic optical element in the holographic        recording material,    -   repeating the adjusting and recording steps until the properties        of each of the plurality of optical elements has been recorded,        and    -   performing the development after the optical properties of all        the optical elements have been recorded and removing the        feedback system when the holographic optical element has been        developed.

In this case, the adjusting and recording steps are performed for eachoptical element of the feedback system. Thus, each optical element is inturn adjusted to achieve a desired optical property of the opticalelement in question. This optical property is then recorded. In order torecord the optical properties of all the optical elements into the sameholographic optical element, the holographic optical recording materialis not developed until all the optical properties have been recorded.

At least one of the plurality of optical elements may be selected fromthe group consisting of:

-   -   spatial filters,    -   gratings,    -   mirrors,    -   Fabry Perot etalons,    -   frequency filters.

These optical elements and their optical properties have been describedabove.

Preferably, the method further comprises the step of positioning theholographic optical element in connection with a laser device, so thatthe holographic optical element and the laser device may cooperate toselect a state having a high temporal and/or spatial coherency.

That is, the holographic optical element may preferably be used toreplace the feedback system in order to provide a compact, cheap, andmechanically stable laser system as described above, and as will befurther described below.

The method may further comprise the step of multiplexing a plurality ofcentre frequencies into the holographic optical element. In anembodiment where the feedback system comprises a grating, this may beperformed in the following way.

The feedback system may be adjusted to select one centre frequency and acorresponding grating may be induced in the holographic optical element.The laser device may then be turned off and the grating be tilted toselect a new centre frequency. When the laser device is turned on again,a new hologram with a new centre frequency may be written into theholographic optical recording material. By repeating this procedure foreach centre frequency, a plurality of frequencies are written into theholographic recording material. When all the desired frequencies havebeen written into the holographic recording material, the holographicoptical element is developed, so as to obtain a holographic opticalelement having all the desired centre frequencies multiplexed into it.

Thus, the method may further comprise the steps of, for each of theplurality of centre frequencies:

-   -   adjusting the feedback system to emit a centre frequency        feedback light beam so that the laser device and the feedback        system cooperate to select a state having a high temporal and/or        spatial coherency, and so that a specific centre frequency is        obtained,    -   recording a holographic optical element in the holographic        recording material,    -   repeating the adjusting and recording steps until each of the        plurality of centre frequencies has been recorded, and    -   performing the development after all the centre frequencies have        been recorded and removing the feedback system when the        holographic optical element has been developed.

The developing step may be performed using a chemical or thermal fixingprocedure. Alternatively, the holographic recording material may be of akind which is ‘self-developing’. In this case the development isperformed automatically and does not require an active act. This isknown per se.

The laser system may advantageously be a compact laser system.

According to a third aspect the invention further provides a compactlaser system for emission of an output light beam, the systemcomprising:

-   -   a laser device for emission of a first light beam, and    -   a holographic optical element being illuminated by at least a        part of the first light beam, thereby causing a feedback light        beam to be emitted from the holographic optical element and        being reinjected into the active gain medium of the laser        device, whereby the laser device and the holographic optical        element cooperate to select a high spatial and/or high temporal        coherent state of the laser device, whereby the laser system is        controlled to emit an output light beam having an improved        spatial and/or temporal coherence.

As described above the laser device may be any suitable kind of laserdevice, such as a gas laser, a semiconductor laser, a superluminescentlaser diode, a dye laser, a Nd-YAG laser, an argon ion laser, a titaniumsapphire laser, an F-center laser, or any other suitable kind of laser.It may also be an array of lasers, said lasers being of any of the typesmentioned above.

The first light beam may be an electromagnetic beam, preferably amonochromatic electromagnetic beam. In case the laser device is a singlelaser, the first light may also be a coherent light beam. In case thelaser device is an array of lasers or another laser device having abroad bandwidth gain medium, the first light beam will in most caseshave a very low degree of coherence.

The holographic optical element is illuminated by at least part of thefirst light beam. It may, of course, be illuminated by all of the firstlight beam. At least part of the holographic optical element may beilluminated, or all of the holographic optical element may beilluminated.

The feedback light beam is emitted from the holographic optical elementin response to the first light beam illuminating the holographic opticalelement. The feedback light beam may be an electromagnetic beam, such asan electromagnetic beam comprising frequencies within the visiblefrequency range, the ultraviolet frequency range, the infrared frequencyrange, the X-ray frequency range, or any other suitable frequency range.The feedback light beam may be e.g. a complete reflection of the firstlight beam. Alternatively, it may be a reflected part of the first lightbeam, such as a part defined by a specific frequency range, a specificpolarisation, a specific spatial mode, etc. Alternatively, the feedbacklight beam may be a beam which is generated by the holographic opticalelement in response to the first light beam.

The feedback light beam is reinjected into the active gain medium of thelaser device. In this way the laser device and the holographic opticalelement cooperate to select a high spatial and/or temporal coherentstate of the laser device. The output light beam from the system willthen have a high spatial and/or temporal coherent state.

In order to obtain a high power, the laser device is often an array oflasers as described above. As mentioned above, this will very oftenresult in a first light beam having a low degree of coherence. Since theoutput light beam has a high spatial and/or temporal coherent state, itthus has an improved spatial and/or temporal coherence as compared tothe first light beam being emitted from the laser device. Thereby, thecompact laser system is adapted for improving the coherency of a highpower laser beam.

The holographic optical element may be adapted to reconstruct anoriginal light beam from a feedback system.

The feedback system may comprise a number of optical elements asdescribed above, and it is preferably operated as described above.

The holographic optical element may, thus, replace the bulky, expensive,and fragile optical elements of the feedback system. Since a holographicoptical element is compact, cheap, and less fragile than most ordinaryoptical elements, the resulting laser system will also be compact,cheap, and less fragile than laser systems having an ordinary feedbacksystem comprising a number of optical elements. Furthermore, theresulting laser system will not be subject to misalignments due to,e.g., vibrations or temperature variations to the same extend that anordinary laser system is.

The feedback system may comprise one or more optical elements selectedfrom the group consisting of:

-   -   spatial filters,    -   gratings,    -   lenses,    -   mirrors,    -   Fabry Perot etalons,    -   frequency filters.

Most of these optical elements as well as their optical properties havebeen described above. The original light beam from the feedback systemwhich the holographic optical element is adapted to reconstruct,preferably comprises information relating to the optical properties ofthe optical elements of the feedback system. The holographic opticalelement is most preferably recorded in such a way that these opticalproperties may be reproduced by the holographic optical element. Thus,the output light beam from the compact laser system may be substantiallyidentical to the output light beam from a more bulky laser system havingan ordinary feedback system.

In case one of the optical elements is a lens, the optical properties tobe reproduced by the holographic optical element preferably compriserefractive index, reflectivity, including internal reflectivity, focallength, radius of curvature, or any other suitable optical properties ofthe lens. The lens may be an ordinary concave or convex lens, or it maybe another kind of refractive optical element, such as a prism.

The holographic optical element may be adapted to, in cooperation withthe laser device, select at least one centre frequency from the firstlight beam. This corresponds to selecting a high temporal coherentstate. However, the exact value of the centre frequency may also beselected in this embodiment. This may e.g. be obtained by recording theholographic optical element using a feedback system which may be tunedso as to select a specific frequency.

The holographic optical element may be adapted to, in cooperation withthe laser device, select a plurality of centre frequencies, each centrefrequency being multiplexed into the holographic optical element.

This may be obtained by consecutively tuning a system as described aboveto each of the plurality of centre frequencies, and recording anddeveloping the holographic optical element in such a way that all thecentre frequencies are multiplexed into the holographic optical element.This procedure will be further described below.

The laser device may comprise a laser array, such as an array of diodelasers, gas lasers, semiconductor lasers, dye lasers, Nd-YAG lasers,argon ion lasers, or any other suitable kind of lasers. Alternatively,it may be a single laser as described above.

The laser device may comprise at least one laser selected from the groupconsisting of:

-   -   broad area lasers,    -   laser diode arrays,    -   laser diode bars,    -   stacked laser arrays.

Broad area lasers and laser diode arrays comprise a number of diodelasers arranged in a row.

Laser diode bars also comprise a number of diode lasers arranged in arow. However, the lasers of a laser diode bar are spatially separated,so that the light sources may be considered as a number of discretepoint sources.

Stacked laser arrays comprise a number of laser diode bars beingstacked, so as to form a two-dimensional array of diode lasers.

The above-mentioned types of lasers all provide an output beam having ahigh power, but a low degree of coherence. For some applications, it maytherefore be necessary to improve the coherency of the output beam. Thismay be done as previously described.

According to a fourth aspect the invention further provides a method ofgenerating an output light beam from a laser system, the laser systemcomprising a laser device and a holographic optical element, the methodcomprising the steps of:

-   -   emitting, by means of the laser device, a first light beam in        such a way that at least part of the holographic optical element        is illuminated by at least part of the first light beam,    -   injecting, by means of the holographic optical element and in        response to the first light beam, a feedback light beam into the        laser device, and    -   outputting, by means of the holographic optical element and in        response to the first light beam, an output light beam from the        laser system, said output light beam having an improved spatial        and/or temporal coherence state.

The first light beam, the feedback light beam, as well as the outputlight beam may be electromagnetic beams as described above.

All of, or at least part of, the holographic optical element may beilluminated by the first light beam, and it may be illuminated by allof, or at least part of, the first light beam.

The feedback light beam may be a fully or a partial reflection of thefirst light beam, or it may be generated by the holographic opticalelement, as described above.

The laser device and the holographic optical element cooperate to selecta state having a high temporal and/or spatial coherence, so that theoutput light beam has an improved spatial and/or temporal coherence ascompared to the first light beam which is initially emitted from thelaser device. This has been described above.

The holographic optical element may reconstruct an original light beamfrom a feedback system. This has already been described.

The feedback system may comprise one or more optical elements selectedfrom the group consisting of:

-   -   spatial filters,    -   gratings,    -   lenses,    -   mirrors,    -   Fabry Perot etalons,    -   frequency filters.

These optical elements as well as their optical properties have beendescribed above.

The method may further comprise the step of, by means of the holographicoptical element in cooperation with the laser device, selecting at leastone centre frequency from the first light beam. As described above thiscorresponds to selecting a state having a high temporal coherence.However, a specific frequency is chosen in this case.

The method may further comprise the step of, by means of the holographicoptical element in cooperation with the laser device, selecting aplurality of centre frequencies, each centre frequency having previouslybeen multiplexed into the holographic optical element. This has alsobeen described above.

According to a fifth aspect the invention further provides a method ofproducing a compact laser system for emission of an output light beam,the method comprising the steps of:

-   -   inserting a holographic recording material into a laser cavity        formed between a laser device and a feedback system,    -   emitting, by means of the laser device, a first light beam, at        least part of said first light beam illuminating at least part        of the feedback system via said holographic recording material,    -   adjusting the feedback system to emit a feedback light beam so        that the laser device and the feedback system cooperate to        select a state having a high temporal and/or spatial coherency,    -   recording a holographic optical element in the holographic        recording material,    -   developing the holographic optical element so that the        holographic optical element is capable of reconstructing the        feedback light beam from the feedback system when said feedback        system is removed, and    -   removing the feedback system.

The laser device may be any suitable kind of laser device as describedabove. The feedback system preferably comprises a number of opticalelements each having specific optical properties.

The first light beam as well as the feedback light beam may beelectromagnetic beams as described above.

The adjusting step is preferably performed by adjusting each of theoptical elements of the feedback system. This may comprise tiltinggratings to the correct angle, e.g. in order to obtain a specificfrequency, positioning spatial filters correctly, e.g. in order toobtain a specific spatial mode, aligning the optical elements, e.g. inorder to optimise the throughput of the system, and/or it may compriseany other suitable kind of adjusting of the feedback system. Theadjusting step results in that the laser device and the feedback system,by means of the feedback light beam, cooperate to select a state havinga high temporal and/or spatial coherency. The adjusting may be performedusing spatial filtering in the Fourier plan.

The holographic recording material may comprise a dichromatic gelatine,a silver bromide (AgBr) solution, photo resist, and/or a photorefractivemedium, such as a lithium niobate crystal.

When the holographic optical recording material has been developed toform the holographic optical element, the holographic optical element iscapable of reconstructing the feedback light beam from the feedbacksystem because the optical properties of the optical elements of thefeedback system have been recorded into the holographic optical element.When the feedback system is removed, the laser device and theholographic optical element may therefore be able to cooperate to selecta state having a high temporal and/or spatial coherency. That is, theoutput light beam emitted from the laser system with the feedback systemremoved will be substantially identical to the output light beam emittedfrom the laser system with the feedback system present instead of theholographic optical element. Thus, as described above, the holographicoptical element may replace the rather bulky, expensive, etc. feedbacksystem, thereby providing a laser system which is compact, cheap, etc.

The method may further comprise the step of multiplexing a plurality ofcentre frequencies into the holographic optical element.

Thus, the method may further comprise the steps of, for each of theplurality of centre frequencies:

-   -   adjusting the feedback system to emit a centre frequency        feedback light beam so that the laser device and the feedback        system cooperate to select a state having a high temporal and/or        spatial coherency, and so that a specific centre frequency is        obtained,    -   recording a holographic optical element in the holographic        recording material,    -   repeating the adjusting and recording steps until each of the        plurality of centre frequencies has been recorded, and    -   performing the development after all the centre frequencies have        been recorded and removing the feedback system when the        holographic optical element has been developed.

This has already been described above.

A laser system according to the present invention may be used for anumber of various applications, such as frequency doubling, coupling oflight into thin core or single mode laser fibres, material processing,the printing industry, biomedical application, and/or any other suitableapplications in which a compact, low cost, and reliable laser system isuseful.

The first, second, third, fourth, and fifth aspects of the presentinvention may each be combined with one or more of the other aspects ofthe present invention.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows the recording of a feedback system into a holographicoptical element,

in FIG. 2, the holographic optical element has been developed and thefeedback system has been removed,

FIG. 3 shows an example of a feedback system comprising a laser diodearray receiving an external feedback from a reflector, and

FIG. 4 shows an example of an output from a laser operatingsimultaneously at several wavelengths.

DETAILED DESCRIPTION OF THE DRAWINGS

In FIG. 1, a laser diode array 1 and an external cavity formed betweenthe laser diode array 1 and the feedback system 3 is shown. The laserarray 1 receives a feedback signal from the feedback system 3. The laserbeam 11 emitted from the laser array 1 forms a twin lobe structure inthe plane of the far field, and one of the lobes, the lobe 5,illuminates the feedback system 3 whereas the other lobe 7 provides anoutput light beam.

A holographic optical recording material 9 is inserted in the path ofthe laser beam 11 between the laser array 1 and the feedback system 3,preferably, the holographic optical recording material 9 is insertedproximate to the laser array 1 emitting the laser beam 11 so that theresulting laser system may be minimised as much as possible. It is,though, emphasised that the holographic optical recording material maybe inserted just after the laser or anywhere in the feedback system.

According to a preferred embodiment of the present invention, thefeedback system 3 is a frequency selective feedback system adjusted toselect a high spatial and/or a high temporal coherent state of the laserarray 1. Having adjusted the laser system to inject a feedback signalinto the laser array 1 to select a high spatial and/or high temporalcoherent state of the laser array 1, the holographic optical recordingmaterial 9 is developed either spontaneously or by a chemical and/orthermal procedure.

In FIG. 2, the optical system comprising the developed holographicoptical element 13 is shown. The feedback system 3 is removed and thedeveloped holographic optical element 13 is inserted in the path of thelaser beam 11 at the same position as the position on which theholographic optical recording material 9 was recorded. The holographicoptical element 13 hereby reconstructs the original signal from thefeedback system (now removed) which is injected into the laser array 1,and in the preferred embodiment mentioned above, the holographic opticalelement 13 and the laser array 1 will select a high spatial and/ortemporal coherent state of the laser array 1 whereby the laser array 1is adapted to emit a high spatial and/or high temporal coherent outputlight beam 7. The holographic optical element may be of a material suchas dichromatic gelatines, silver bromide, photo resist, or aphoto-refractive medium, such as a lithium niobate crystal.

FIG. 3 shows an example of a frequency selective feedback systemaccording to the preferred embodiment mentioned above. In FIG. 3, theentire laser beam 11 illuminates the holographic optical recordingmaterial.

The laser array 30 used in the embodiment shown in FIG. 3 is a SDL-2432GaAlAs broad area laser with a threshold of 0.29 Amps and a maximumoutput power of 0.5 Watts at a drive current of 0.75 Amps. The lasingwavelength at 20.0° C. is 813.5 nm and the emitting junction 31 is 1×100μm.

The light emitted from the laser array 30 is collimated with a ThorlabC230TM-B lens 33 with an effective focal length of 4.5 mm and anumerical aperture of 0.55. A beam splitter 51 is inserted in the systemto couple a part of the beam 55 out for beam diagnostics. A cylindricallens 35 with a focal length of 150 mm is inserted to collimate the beam.

Furthermore, an etalon may be provided causing the feedback to befrequency selective, the etalon may be a Fabry-Perot etalon with afinesse of approx. 17. Alternatively, an etalon having a lower finesse,such as approx. 2.6 may be used. The etalon, thus, improves the temporalcoherence of the output beam. All lenses and etalons have a broad bandanti-reflection coating (R<1 percent) in order to minimise the loss ofthe external cavity. Furthermore, the exit face of the laser may haveantireflection coating to improve the influence from the feedbacksystem. The external cavity is terminated by a grating 37 and areflector 39, the reflector 39 being a mirror or in a preferredembodiment, a four-wave mixing non-linear medium, such as a GaAlAscrystal arranged in a self-pumped configuration.

A spatial filter 41 is inserted in the lobe 45 in the plane of thepseudo far field 43, whereby only selected spatial modes of the lobe 45are directed towards the grating and the reflector. In the other lobe47, a mirror 49 is inserted for coupling an output light beam out of thesystem.

To record this system into a holographic optical recording medium, theholographic optical recording medium is positioned in the laser beam 11after the collimating lens 33. When the system is adjusted to emit anoutput light beam having a high spatial and/or temporal coherence, theholographic optical recording medium is developed and the feedbacksystem comprising lenses 34, 35, beam splitter 51, mirror 49, spatialfilter 41, grating 37 and reflector 39, may be removed.

Hereby, the complex optical system shown in FIG. 3 may be replaced by asingle holographic optical element, whereby the size of the opticalsystem is dramatically reduced. Furthermore, adjustment of gratings,spatial filters, etalons, etc, is avoided during operation or start-upof the system, since this adjustment is inherently present in theholographic optical element. The holographic optical element need onlyto be inserted at the same position in relation to the laser array orlaser device as the position of holographic optical recording mediumduring recording of the medium. This reduces the overall cost andcomplexity of the system, and for example very fragile and expensivecomponents, such as non-linear mediums, may be replaced by a low-costand reliable holographic optical element, and the fragile and expensivemediums may be used to record a plurality of holographic opticalelements in controlled environments. A further advantage of thedescribed system is that all components may be integrated on a singlechip.

The frequency selective feedback system may, as described in connectionwith FIG. 3, be a grating based feedback system comprising a mirror, aspatial filter, grating and lenses. In an alternative preferredembodiment, the frequency selective feedback system may comprisegratings, lenses and a spatial filter. In a still further preferredembodiment of the invention, the frequency selective feedback systemcomprises non-linear four-wave mixing in combination with gratings,spatial filters and frequency filters.

The grating is a frequency filter which only refracts a limited numberof frequencies which then interact with the reflector. By tilting thegrating so that the feedback beam is adjusted to match the lasingwavelength of a spatial mode with high gain in the laser array, thefrequency of the output beam is tuned. Similarly, an etalon incombination with a non-linear four-wave mixing medium passes only alimited number of frequencies so that, as the reflectivity of thenon-linear four-wave mixing medium increases, the spectrum narrows downsignificantly. Single spatial mode operation can be achieved if theorientation of the etalon is adjusted so the wavelength for peaktransmission match the lasing wavelengths of a spatial mode with highgain or centre frequencies of the laser array.

Advantage of the possibility of tilting the grating 37 or the etalon(not shown in FIG. 3) for selecting a specific lasing wavelength of thelaser array may be taken when recording a holographic optical recordingmedium.

By having the holographic optical recording medium positioned in theoptical system, the tilting angle of the grating may be varied. At afirst centre frequency, a first holographic grating according to thetilting angle of the grating may be recorded in the holographic opticalrecording medium. The laser array is then turned off and the grating istilted to select a new centre frequency. When the laser is turned on, anew hologram with a new centre frequency, a new holographic grating, iswritten into the holographic optical recording medium. By using thisprocedure at a plurality of centre frequencies, a plurality ofholographic gratings are written into the holographic optical recordingmedium. When a predetermined number of frequencies have been writteninto the material, the material is developed and afterwards theholographic optical element may be used to provide an output asillustrated in FIG. 4, where a number of four centre frequencies hasbeen written into the holographic optical recording medium beforedeveloping the medium. The configuration used to obtain the output shownin FIG. 4 is a configuration according to the configuration shown inFIG. 2.

The possibility of multiplexing a number of centre frequencies into anholographic optical element enables the laser array or any other laserdevice to operate simultaneously at several wavelengths. This is anespecially important feature in high capacity telecommunication systems.

1. An optical system for emission of an output light beam, wherein aholographic optical element reproduces the optical properties of aplurality of optical elements, the plurality of optical elements forminga feedback system being adapted to cooperate with a laser device toselect a high temporal and/or spatial coherent state of the laserdevice.
 2. An optical system according to claim 1, wherein at least oneof the plurality of optical elements is selected from the groupconsisting of: spatial filters, gratings, mirrors, Fabry Perot etalons,frequency filters.
 3. A method of producing an optical system foremission of an output beam, the method comprising the steps of:inserting a holographic recording material into an external cavityformed between a laser device and a feedback system, said feedbacksystem comprising a plurality of optical elements, emitting, by means ofthe laser device, a first light beam, at least part of said first lightbeam illuminating at least part of the feedback system via saidholographic recording material, adjusting the feedback system so thatthe laser device and the feedback system cooperate to select a statehaving a high temporal and/or spatial coherence, recording a holographicoptical element in the holographic recording material, developing theholographic optical element so that the holographic optical element isadapted to reproduce the optical properties of the plurality of opticalelement when said feedback system is removed, and removing the feedbacksystem.
 4. A method according to claim 3, the method comprising thesteps of, for each of the optical elements: adjusting the feedbacksystem so that the laser device and the feedback system cooperate toselect a state having a high temporal and/or spatial coherency,recording a holographic optical element in the holographic recordingmaterial, repeating the adjusting and recording steps until theproperties of eachof the plurality of optical elements has beenrecorded, and performing the development after the optical properties ofall the optical elements have been recorded and removing the feedbacksystem when the holographic optical element has been developed.
 5. Amethod according to claim 3 or 4, wherein at least one of the pluralityof optical elements is selected from the group consisting of: spatialfilters, gratings, mirrors, Fabry Perot etalons, frequency filters.
 6. Amethod according to any of claims 3-5, further comprising the step ofpositioning the holographic optical element in connection with a laserdevice, so that the holographic optical element and the laser device maycooperate to select a state having a high temporal and/or spatialcoherency.
 7. A method according to any of claims 4-6, furthercomprising the step of multiplexing a plurality of centre frequenciesinto the holographic optical element.
 8. A method according to claim 7,the method further comprising the steps of, for each of the plurality ofcentre frequencies: adjusting the feedback system to emit a centrefrequency feedback light beam so that the laser device and the feedbacksystem cooperate to select a state having a high temporal and/or spatialcoherency, and so that a specific centre frequency is obtained,recording a holographic optical element in the holographic recordingmaterial, repeating the adjusting and recording steps until each of theplurality of centre frequencies has been recorded, and performing thedevelopment after all the centre frequencies have been recorded andremoving the feedback system when the holographic optical element hasbeen developed.
 9. A method according to any of claims 3-8, wherein thedeveloping step is performed using a chemical or thermal fixingprocedure.
 10. A method according to any of claims 3-9, wherein thelaser system is a compact laser system.
 11. A compact laser system foremission of an output light beam, the system comprising: alaser devicefor emission of a first light beam, and a holographic optical elementbeing illuminated by at least a part of the first light beam, therebycausing a feedback light beam to be emitted from the holographic opticalelement and being reinjected into the active gain medium of the laserdevice, whereby the laser device and the holographic optical elementcooperate to select a high spatial and/or high temporal coherent stateof the laser device, whereby the laser system is controlled to emit anoutput light beam having an improved spatial and/or temporal coherence.12. A laser system according to claim11, wherein the holographic opticalelement is adapted to reconstruct an original light beam from a feedbacksystem.
 13. A laser system according to claim 12, wherein the feedbacksystem comprises one or more optical elements selected from the groupconsisting of: spatial filters, gratings, lenses, mirrors, Fabry Perotetalons, frequency filters.
 14. A laser system according to any ofclaims 11-13, wherein the holographic optical element is adapted to, incoopreation with the laser device, select at least one centre frequencyfrom the first light beam.
 15. A laser system according to any of claims11-14, wherein the holographic optical element is adapted to, incooperation with the laser device, select a plurality of centrefrequencies, each centre frequency being multiplexed into theholographic optical element.
 16. A laser system according to any ofclaims 11-15, wherein the laser device comprises a laser array.
 17. Alaser system according to any of claims 11-16, wherein the laser devicecomprises at least one laser selected from the group consisting of:broad area lasers, laser diode arrays, laser diode bars, stacked laserarrays.
 18. A method of generating an output light beam from a lasersystem comprising a laser device and a holographic optical element, themethod comprising the steps of: emitting, by means of the laser device,a first light beam in such a way tha t at least part of the holographicoptical element is illuminated by at least part of the first light beam,injecting, by means of the holographic optical element and in responseto the first light beam, a feedback light beam into the laser device,and outputting, by means of the holographic optical element and inresponse to the first light beam, an output light beam from the lasersystem, said output light beam having an imptoved spatial and/ortemporal coherence state.
 19. A method according to claim 18, whereinthe feedback system comprises one or more optical elements selected fromthe groupt consisting of: spatial filters, gratings, lenses, mirrors,Fabry Perot etalons, frequency filters.
 21. A method according to any ofclaims 18-20, further comprising the steps of, by means of theholographic optical element in cooperation with the laser device,selecting at least one centre frequency from the first light beam.
 22. Amethod according to any of claim 18-21, further comprising the steps of,by menas of the holographic optical element in cooperation with thelaser device, selecting at least one centre frequency from the firstlight beam.
 23. A method of producing a compact laser system foremission of an output light beam, the method comprising the steps of:inserting a holographic recording material into a laser cavity formedbetween a laser device and a feedback system, emitting, by means of telaser device, a first light beam, at least part of said first light beamilluminating at least part of the feedback system via said holographicrecording material, adjusting the feedback system to emit a feedbacklight beam so that the laser device and the feedback system cooperate toselect a state having a high temporal and/or spatial coherency,recording a holographic optical element in the holographic recordingmaterial, developing the holographic optical element so that theholographic optical element is capable of reconstructing the feedbacklight beam from the feedback system when said feedback system isremoved, and removing the feedback system.
 24. A method according toclaim 23, further comprising the step of multiplexing a plurality ofcentre frequencies into the holographic optical element.
 25. A methodaccording to claim 24, the method further comprising the steps of, foreach of the plurality of centre frequencies: adjusting the feedbacksystem to emit a centre frequency feedback light beam so that the laserdevice and the feedback system cooperate to select a state having a hightemporal and/or spatial coherency, and so that a specific centrefrequency is obtained, recording a holographic optical element in theholographic recording material, repeating the adjusting and recordingsteps until each of the plurality of centre frequencies has beenrecorded, and performing the development after all the centrefrequencies have been recorded and removing the feedback system when theholographic optical element has been developed.
 26. A method accordingto any of claims 23-25, wherein the developing step is performed using achemical or thermal fixing procedure.